Photoreceptors are responsible for transforming light energy impinging on the retina into neural signals, and therefore information contained in the stimulus but not transmitted by the photoreceptors is lost to the organism. For this reason, knowledge of the response characteristics of the photoreceptors not only provides information about the receptors themselves, but is also a necessary prerequisite to an understanding of more central processing in the visual system. Intraretinal microelectrode recording techniques that provide access to the receptor response, in particular in late receptor potential, enable us to study primate photoreceptor signal processing. By using cynomolgus macaque monkeys, which have human-like visual systems, we can address questions pertinent to human visual perception. The identification of psychophysical phenomena determined at the photoreceptor-horizontal cell level will provide a means of studying retinal diseases in man with psychophysical techniques. Quantitative analysis of the LRP will enhance the electroretinagram (ERG) as a diagnostic tool. Accordingly, the foveal local ERG will be used to investigate the role of the photoreceptors in various perceptual phenomena by using stimuli capable of enhancing the late receptor potential. Recent advances in microelectrode technology will be applied in an attempt to achieve intracellular recording from primate cones, and in conjunctions with this technical development we will study the temporal sensitivity of primate ganglion cells.